MDL-62714 theme_boost: Course visibility in navigation bar

Added check in navbar.mustache to add dimmed_text class if needed.
Added is_hidden() in navigationlib.php to return if the class needs
to be added.

lib/navigationlib.php

@@ -686,6 +686,15 @@ class navigation_node implements renderable {
         return !empty($this->action);
     }
 
+    /**
+     * Used to easily determine if this link in the breadcrumbs is hidden.
+     *
+     * @return boolean
+     */
+    public function is_hidden() {
+        return $this->hidden;
+    }
+
     /**
      * Gets the CSS class to add to this node to describe its type
      *